In the prior art, many techniques have been developed for determining the composition of a sample. One of the more common techniques is spectroscopy wherein electromagnetic energy emitted from a sample is measured and evaluated to determine the elements contained in the sample. While there are many types of spectroscopic measurement methods, the subject invention is particularly adapted for use with X-ray detectors. In the latter technique, a detector is provided which senses X-rays emanating from a sample.
Using an X-ray fluorescent analyzer, a spectrum associated with the sample can be generated. By analyzing the spectral data, the composition of the sample can be determined.
X-ray detection equipment is typically designed to operate in conjunction with electron microscopes, such as scanning electron microscopes or transmission electron microscopes. The specimen chamber in these electron microscopes must be operated in a vacuum. This requirement imposes rather stringent design criteria on X-ray detection devices since they must be compatible with a vacuum environment.
In the prior art, a number of X-ray detection devices have been developed for use with electron microscopes. The devices are generally provided with an elongated tubular member or probe, which is typically connected to some form of frame for housing the hardware of the device. The opposed free end of the tubular member is received in the specimen chamber of the electron microscope, and is sealed with a radiation passing window. The interior of the tubular member contains a channel for receiving the X-ray radiation entering the probe through the window. An X-ray sensor, such as a lithium-drifted silicon device, is mounted in the channel of the probe, behind the window. Since the sensor requires a vacuum to operate, the channel of the probe must be sealed and evacuated.
As mentioned above, the free end of the probe is provided with some form of window to permit the X-ray radiation to enter the channel and reach the sensor. More particularly, a relatively thin window, formed for example, from aluminum foil, is provided to permit a high percentage of X-rays, emanating from the sample, to enter the channel. Unfortunately, a thin aluminum window, while effective for passing a large amount of radiation, is structually weak. This weakness would pose no difficulties if the window were never subjected to the strains of air pressure. However, in normal procedures, each time a new sample is introduced into the electron microscope, the specimen chamber must be exposed to full air pressure conditions. Since a vacuum is present in the channel of the probe, the thin film window is subjected to an extreme pressure differential when the microscope chamber is exposed to atmospheric pressure. This pressure differential will result in the rupture of the thin film window. If the thin film window is ruptured, the shock of the abrupt pressure change in the probe can damage the sensor.
Accordingly, a means must be provided to prevent the rupture of the thin film window. This object is achieved in some prior art detectors by providing a second, thicker and stronger window, which will resist collapse when subjected to normal air pressure. The thicker window may be in the form of a beryllium foil, which will transmit a portion of the X-ray spectrum, particularly at higher energy levels. Since the beryllium window will pass some radiation, it can be used in many measurement situations. However, many test techniques require that lower level energy radiation be detected, such that a beryllium window alone is insufficient.
In the prior art devices which rely on a two-window construction, a means must be provided for selectively aligning one of the two windows with the channel of the probe. In operation, the thicker beryllium window is initially aligned with the channel of the probe. After the sample has been placed in the electron microscope and the specimen chamber is evacuated, a mechanism must operate to move the thin aluminum film window into alignment with the channel. When the testing is complete, the beryllium window is moved back into alignment with the channel, prior to the pressurization of the chamber, thereby preventing the rupture of the thin film window.
The mechanisms used in the prior art were capable of moving the thin film window into and out of alignment with the channel. However, the latter mechanisms tended to be relatively cumbersome, which inhibited optimum measurement capability. For example, one known device included the use of a large outer tube having both windows mounted thereon. The outer tube was disposed around the probe and mounted for rotational movement along an axis offset from the axis of the probe. By rotating the outer tube, the windows could be brought into selective alignment with the channel.
Unfortunately, the use of the large outer tube added significantly to the total diameter of the probe. This extra size created some difficulties. More particularly, many electron microscopes could not accommodate a probe having a large diameter. In addition, because of the geometry of the specimen chamber, it was difficult to move a large diameter probe into close proximity with the sample. Since radiation levels fall off as function of the distance squared, it is highly desirable to be able to position the probe as close to the sample as possible.
Some devices found in the prior art are provided with only a single, thin window. In the latter devices, the thin film window is protected by a gate valve. In use, when the specimen chamber is evacuated, the gate in the valve is retracted, exposing the window to permit radiation to pass into the probe towards the sensor. Thus, a functional window support can be manufactured which does not include a second, thicker window. However, a second, thicker window provides enhanced versatility by permitting sensing in some measurement situations.
Accordingly, it is an object of the subject invention to provide a new and improved mechanism for selectively aligning at least one radiation passing window with the channel of a detector, that overcomes the shortcomings of the prior art devices.
It is another object of the subject invention to provide a new and improved mechanism which is relatively compact in configuration, permitting the tubular probe to be moved relatively close to the sample for enhanced sensitivity.
It is a further object of the subject invention to provide a new and improved mechanism, for selectively aligning one of two different windows with the channel, which is operable from a point spaced from the end of the probe.
It is still another object of the subject invention to provide a new and improved mechamism for selectively aligning one of two different windows with the channel of a probe wherein at least one of said windows is readily replaceable.
It is still a further object of the subject invention to provide a new and improved mechanism for selectively aligning one of two different windows with the channel of a probe which includes an interlock means to prevent the relatively fragile thin film window from being moved into alignment with the channel until a vacuum has been established in the specimen chamber of the microscope.